Tim that works great and thank you for pointing it out, especially when the arc is part of a linestring.
A dedicated arc measure tool would still be helpful when working with PDFs and images. Something that would allow you to click in 3 spots and get a radius and arc length. We use this a lot with value engineering. My boss will come into my office and starting asking "What's the length of this? "Whats the radius of that curb?". The measure tools are very useful for quick analysis and then you don't have to worry about deleting them out.
Thanks!